Do Low Calorie Diets Increase Longevity?

Want to live a long time? Who doesn't? Quite a number of smart people have staked their hopes for long life on restricting their calorie intake. It's an idea that rests on a not-inconsiderable body of evidence. But some new developments suggest it may be a questionable strategy.

The idea that a diet containing a very modest energy content (measured in calories or kilojoules) could dramatically prolong lifespan is based on one of the most replicated findings in all of biology.

As long ago as 1935, researchers reported that lab rats fed dramatically-restricted diets lived up to twice as long as rats allowed unrestricted access to food. The lifespan-extending effects of dietary restriction appear to come about by delaying the onset of aging and slowing the cellular damage and onset of diseases like cancers that naturally occur with age.

Since then, similar results have been obtained in mice, Rhesus monkeys, zebra fish, vinegar flies (Drosophila melanogaster), the nematode worm (Caenorhybditis elegans) and even yeast. The fact that dietary restriction prolonged lifespan and slowed aging in such a variety of organisms strongly suggested that the underlying mechanisms are shared by all animals and possibly even all eukaryotes -- the large group of complex organisms that includes animals, plants and fungi.

It didn't take long for people to spot a potential route to slowing aging and putting off the inevitable. Roy Walford championed what he called CRON -- Calorie Restriction with Optimal Nutrition -- in Beyond The 120-Year Diet. He argued that cutting dietary energy while maintaining micronutrient intake would prolong normal lifespan to well over a century, reducing the incidence of the major diseases of aging.

We are not talking here about avoiding overeating or gluttony, but rather about cutting 20 to 40 percent from the energy content normally recommended to maintain a healthy body mass. Walford was among the founders and the chief scientific luminary of the Caloric Restriction Society, the most prominent of many organizations devoted to promoting health through dietary restriction. There are books, and Internet forums and even online tools like Interactive Diet Planner and the puntastic CRON-O-Meter calorie tracking software.

While dietary restriction seems to have a lot of scientific evidence in its favor, that evidence remains largely circumstantial. It takes decades to verify predictions of dramatically-enhanced longevity. And long-term randomized controlled trials are impossible for a restrictive diet that, according to some detractors, promotes "a longer life not worth living."

So the case for dietary restriction rests largely on several short-term studies in humans and the evidence that dietary restriction promotes longevity in a wide variety of organisms.

The problem is that a great many studies in a variety of animals show little or no positive effects of dietary restriction on lifespan or aging. In a recent meta analysis published in Aging Cell, The University of Otago's Dr. Shinichi Nakagawa and his collaborators showed that while dietary restriction significantly increased lifespan across 36 species and 145 studies, the effects were less than straightforward.

First, dietary restriction had more dramatic positive effects in laboratory "model" organisms -- like Drosophila, mice, rats and C. elegans -- than in non-model species. The truly eye-catching studies in the big journals like Nature and Science are more often on the model species, whereas the null results on animals that fewer people are interested in tend to be in specialist journals.

On top of this, dietary restriction tends to have larger positive effects on females than on males. This makes sense because a constrained diet probably slows down costly female reproduction (gestation and egg-laying) more dramatically than less-costly male reproduction. The costs of reproduction are an important determinant of aging and longevity.

Last, where dietary restriction had positive effects, these were more closely related to restricted protein intake rather than restricted overall energy (caloric) content.

Nakagawa's findings echo what Sydney University's Stephen J. Simpson and Massey University's David Raubenheimer said back in 2007. That while there is plenty of evidence that dietary restriction has positive effects in lab animals, the most important details have yet to be worked out. These include whether the restricted diets work by eliminating calories or by avoiding costly excesses of protein or other nutrients.

The fact that dietary restriction tends to work most dramatically in laboratory models suggests the effect might be more about adaptation to laboratory conditions than a general response shared by all animals. Humans, with agriculture, technology and the ability to escape from hunger, might be more similar to lab rats than wild mammals. But then again they might not.

This is an area of research that has a long way to go yet, but things are certainly being shaken up, and that can only be a good thing if we are to gather useful knowledge about how to delay aging and prolong life. I am sure the most cult-like practitioners of caloric restriction won't change their lifestyle, but it is possible they've been reducing the wrong nutrients.

For now, I see no reason to put myself on a dramatic calorie-restricted or protein-reduced diet. Especially given what Simpson and Raubenheimer's research tells us about the importance of protein in avoiding overeating.